what is the point of having the same object in multiple layers?
Camera:

If you have the camera in multiple layers, you can always see where it is positioned.

Lighting:

If you have lights in multiple layers, you can see all your objects under the same conditions.

Meshes:

If you have meshes in multiple layers, you might not have weird shadows caused by an object that isn't visible.

These are only a few; render layers are a very powerful tool and have many functionalities.